Effects of Torrefaction and Densification on Devolatilization Kinetics and Gasification Performance of Switchgrass

Abstract

The overall goal of this research was to investigate effects of pretreatments (torrefaction at 230 and 270&degC, densification, and combined torrefaction and densification) on biomass properties, devolatilization kinetics and gasification performance of switchgrass. Devolatilization kinetics was determined at three heating rates (10, 30 and 50&degC min-1) in inert (nitrogen) and oxidizing (air) atmospheres using a thermogravimetric analyzer. Gasification performance were evaluated at three gasification temperatures (700, 800 and 900&degC) using an externally-heated fixed&ndashbed reactor with air at an equivalence ratio (ER) of 0.3. Devolatilization study showed that switchgrass torrefied at 270&degC had the highest carbon (C) and the lowest hydrogen (H) and oxygen (O) contents (59.16, 4.67, and 34.53% d.b., respectively). This resulted in the lowest atomic O/C (0.44) and H/C (0.95) ratios and the highest higher heating value (27.11 MJ kg-1, d.b.). Combined torrefaction and densification of switchgrass resulted in the least volatile and the highest ash and fixed carbon contents (62.63, 5.91, and 31.45% d.b., respectively). Combined torrefied and densified switchgrass had the highest rate of devolatilization in both atmospheres as evidenced by the largest rate of weight loss peaks (34 and 44 mg min-1 in inert and oxidizing atmospheres, respectively), the lowest start and end temperatures of the rate of weight loss peak (250-300 and 230-310�C in inert and oxidizing atmospheres, respectively). Gasification study showed that bulk density of combined torrefied and densified switchgrass was the highest (598.17 kg m-3, d.b.) requiring less space to store and transport. Pretreatments of switchgrass and gasification temperatures had significant effects on gasification performance. Among all pretreatments, gasification of combined torrefied and densified switchgrass resulted in the highest yields of H2 (0.03 kg/kg biomass) and CO (0.71 kg/kg biomass), highest syngas LHV (4.97 MJ Nm-3), CCE (90.56%), and CGE (68.69%) at the gasification temperature of 900&degC, which show that combined torrefaction and densification significantly improved gasification performance of switchgrass.